The influence of interparticle spacing on cyclic deformation and fatigue crack propagation in an aluminum-4 Pct copper alloy

A high purity Al-4 pct Cu alloy has been overaged for two different times at 400°C giving interparticle spacings (λ) of about 0.53 and 1.37 μm. Cyclic plasticity of the alloy with the smaller interparticle spacing can be explained in terms of plastic deformation behavior controlled by the structure whereas that for the alloy with the larger interparticle spacing is controlled by the matrix. The fatigue lives of the weaker alloy (λ = 1.37 μm) may be accurately predicted using the models of Coffin-Manson and Tomkins, however, these models are not applicable to the stronger alloy (λ = 0.53 μm). It was found that the crack tip opening displacement at the threshold stress intensity range (ΔK_th) was equivalent to the interparticle spacing. ΔK_th is related to the cyclic yield stress, σ_cy and the interparticle spacing in the following manner: ΔK_th ≈ (2 Eλσ_cy)^1/2, whereE is the modulus of elasticity. In the present case, the term λσ_cy is constant, giving the impression that ΔK_th is independent of the mechanical properties and microstructure. At very low growth rates, however, the fatigue crack growth is independent of these parameters and also the method of cyclic deformation. A transition to higher crack growth rates occurs when the plastic zone size reaches approximately one-seventh of the specimen thickness, allowing a nonplanar crack front to be developed. The value of the stress intensity range (ΔK_T) at this transition was found to be dependent upon the interparticle spacing according to the relation: ΔK_Tλ = 9.6 Pa-m^3/2. Formerly Lecturer and Research Associate, Department of Mechanical Engineering, University of Waterloo     

Intrinsic fatigue crack growth

The influences of microstructure and deformation mode on inert environment intrinsic fatigue crack propagation were investigated for Al-Li-Cu-Mg alloys AA2090, AA8090, and X2095 compared to AA2024. The amount of coherent shearable δ (Al₃Li) precipitates and extent of localized planar slip deformation were reduced by composition (increased Cu/Li in X2095) and heat treatment (double aging of AA8090). Intrinsic growth rates, obtained at high constantK _max to minimize crack closure and in vacuum to eliminate any environmental effect, were alloy dependent;da/dN varied up to tenfold based on applied ΔK or ΔK/E. When compared based on a crack tip cyclic strain or opening displacement parameter (ΔK/(σ_ys E)^1/2), growth rates were equivalent for all alloys except X2095-T8 which exhibited unique fatigue crack growth resistance. Tortuous fatigue crack profiles and large fracture surface facets were observed for each Al-Li alloy independent of the precipitates present, particularly δ, and the localized slip deformation structure. Reduced fatigue crack propagation rates for X2095 in vacuum are not explained by either residual crack closure or slip reversibility arguments; the origin of apparent slip band facets in a homogeneous slip alloy is unclear. Better understanding of crack tip damage accumulation and fracture surface facet crystallography is required for Al-Li alloys with varying slip localization.     

Fatigue crack propagation in aluminum-base copper alloys

Fatigue crack propagation ratesda/dN in binary Al alloys with 3.6 wt pct Cu and 6.3 wt pct Cu and commercial 2024 aged at 21°C were compared with 99.95⁺ wt pct aluminum. Omitting an anomalous region at lowΔK, the extrapolated rates for “pure” aluminum are more than 100 times greater than those in the three alloys at the same ΔK. The data for the alloys fit into a single scatter band of a factor of three. It was suggested thatda/dN varies inversely with the square of the strength of the alloy but that another parameter related to the fatigue crack propagation energy per unit area is also important. Theda/dN vs ΔK curves were determined for 3.6 wt pct Cu single crystals aged seven days at 21°C which containGP zones and two and seven days at 160°C which contain mixtures ofθ′ andθ′’. No systematic variation of (da/dN _Δ with crystallographic orientation was discerned, but the naturally aged specimen had a strong orientation dependence on crack initiation. At low ΔK 21°C aged specimens gave the lowestda/dN while at high ΔK the warm aged specimens gave the lower values ofda/dN. Measurement ofda/dN vs ΔK curves were conducted on specimens of 3.6 wt pct Cu with 1 mm equiaxed grains aged for various times at 130°C, 160°C, and 190°C. All warm aged specimens experienced brittle intergranular fracture at sufficiently high ΔK. The transition ΔK where intergranular fracture first appears is inversely proportional to the aging temperature. The change of fracture mode from intra to intergranular occurs gradually over a broad range of ΔK which shifts to lower ΔK with increase in aging temperature. This research was supportd by U.S. Air Force Office of Scientific Research, Office of Aerospace REsearch, Grant No. AF-AFOSR-73-2431.     

Fatigue crack propagation in 99.99+ and 1100 aluminum at 298 and 77 k

The fatigue crack propagation rate,dc/dN, in cold-rolled and annealed 99.99+ Al is about 80 times slower at 77 K than at 298 K. In annealed 1100 Al which contains constituent particles,dc/dN decreases by a factor of 20 on cooling from 298 to 77 K. At 298 and 77 K, annealed 99.99⁺ Al and 1100 Al cyclically harden but the amount is greater at 77 K. Cold-rolled 99.99⁺ Al cyclically hardens at 77 K but cyclically softens at 298 K. The much slower fatigue crack propagation rate at 77 K in aluminum is attributed in part to the increase in cyclic yield stress, σ_y′, on cooling. At 77 K the high rate of work hardening at large strains is also thought to result in high plastic work per unit area of fatigue crack thereby reducing the fatigue crack propagation rate. Rice’s theory for a Mode I plane stress crack predicts the measured plastic zone size if the local stress corresponding to zero plastic strain in the cyclic stress-strain curve is employed in the formula.     

The effects of temperature on fatigue crack propagation in linepipe steel

Fatigue crack propagation rates in an A.P.I. 5L Grade steel were investigated by means of constant deflection amplitude bending fatigue tests at 640 c.p.m. on single edge notched specimens at — 50, — 10, 20 and 70°C in argon. The data were evaluated in terms of the crack propagation rate (da/dN) as a function of the stress intensity range (Δ/K), according toda/dN = ΔK ^m . It was found that dynamic strain aging has a major influence on fatigue crack propagation, resulting in a maximum of the crack propagation rate at room temperature. Similarly, the cyclic plastic zone size is a maximum at room temperature. D. H. Andreasen, formerly with the Department of Mining and Metallurgy, University of Alberta, Edmonton, Alberta, Canada.     

The growth of small fatigue cracks in A286 steel

Fatigue crack propagation in high-strength A286 steel was studied by comparing crack growth rates determined from: (1) conventional long-crack propagation tests, (2) closure-free long-crack tests at constant K_max, and (3) small-crack propagation tests. Small-crack growth rates were measured by following the growth of surface cracks in samples cycled from near-zero stress to 0.5 or 0.8σ_y. While most of the surface cracks became dormant shortly after nucleation, some grew into long cracks, and some of these propagated at cyclic stress intensities below the long-crack threshold, ΔK_th (or ΔK _th ^eff , the threshold cyclic stress intensity after crack closure effects have been removed). Surface cracks grew more rapidly than long cracks at the same ΔKor ΔK_eff. The small-crack effect disappeared when the crack-tip plastic zone size became greater than the grain size. The results show that the absence of crack closure is only one of several factors that influence short-crack growth in A286 steel. Both peak stress and microstructural effects are important. Microstructural effects are apparently responsible for subthreshold crack growth; the cracks that grow at ΔK < ΔK _th ^eff form and grow in statistically weak regions of the microstructure.     

Effect of hydrogen on fatigue crack propagation in vanadium

The influence of hydrogen on fatigue crack propagation in unalloyed vanadium and several hydrogen-charged vanadium alloys has been investigated. The Paris-Erdogan equation,da/dN =C(Δ.K)^m, was approximately obeyed for all alloys. Crack growth rates were lowest in vanadium and dilute vanadium-hydrogen alloys, and were not very sensitive to volume fraction of hydrides in more concentrated alloys. The crack growth exponent,m, is inversely proportional to the cyclic strain hardening rate,n′, and the rate constantC is inversely proportional to the square of the ultimate tensile stress, σ_UTS: Metallographic examination showed hydride reorientation and growth in the originally hydrided alloys. No stress-induced hydrides were observed in V-H solid-solution alloys. Fractures in hydrided materials exhibited cleavage-like features, while striations were noted in unalloyed vanadium and dilute solid-solution alloys.     

Initiation and growth of small fatigue cracks in a Ni-base superalloy

This article reports research on the initiation and growth of small fatigue cracks in a nickel-base superalloy (produced commercially by INCO as INCOLOY* 908) at 298 and 77 K. The experimental samples were square-bar specimens with polished surfaces, loaded in fourpoint bending. The crack initiation sites, crack growth rates, and microstructural crack paths were determined, as was the large-crack growth behavior, both at constant load ratio (R) and at constant maximum stress intensity (K _max). Small surface cracks initiated predominantly at (Nb,Ti)_xC_y, inclusion particles, and, less frequently, at grain boundaries. Small cracks grew predominantly along {111} planes in individual grains and were perturbed or arrested at grain boundaries. For values of ΔK above the large-crack threshold, ΔK _th, the average rate of smallcrack growth was reasonably close to that of large cracks tested under closure-free conditions. However, short-crack growth rates varied widely, reflecting the local heterogeneity of the microstructure. The threshold cyclic stress (Δσ_th) and the threshold cyclic stress intensity (ΔKσ_th) for small surface cracks were measured as functions of the crack size, 2c. The results suggest that a combination of the fatigue endurance limit and the threshold stress intensity for closure-free growth of large cracks can be used to define a fatigue-safe load regime. formerly with Lawrence Berkeley Laboratory     

Fatigue crack propagation in carburized high alloy bearing steels

Fatigue cracks were propagated through carburized cases in M-50NiL (0.1 C,4 Mo, 4 Cr, 1.3 V, 3.5 Ni) and CBS-1000M (0.1 C, 4.5 Mo, 1 Cr, 0.5 V, 3 Ni) steels at constant stress intensity ranges, ΔK, and at a constant cyclic peak load. Residual compressive stresses of the order of 140 MPa (20 Ksi) were developed in the M-50NiL cases, and in tests carried out at constant ΔK values it was observed that the fatigue crack propagation rates,da/dN, slowed significantly. In some tests, at constant peak loads, cracks were stopped in regions with high compressive stresses. The residual stresses in the cases in CBS-1000M steel were predominantly tensile, probably because of the presence of high retained austenite contents, andda/dN was accelerated in these cases. The effects of residual stress on the fatigue crack propagation rates are interpreted in terms of a pinched clothespin model in which the residual stresses introduce an internal stress intensity, K_i where K_i, = σ_id _i ^1/2 (σ_i = internal stress, d_i = characteristic distance associated with the internal stress distribution). The effective stress intensity becomes K_e = K_a + K_i where K_a is the applied stress intensity. Values of K_i were calculated as a function of distance from the surface using experimental measurements of σ_i and a value of d_i = 11 mm (0.43 inch). The resultant values of K_e were taken to be equivalent to effective ΔK values, andda/dN was determined at each point from experimental measurements of fatigue crack propagation obtained separately for the case and core materials. A reasonably good fit was obtained with data for crack growth at a constant ΔK and at a constant cyclic peak load. The carburized case depths were approximately 4 mm, and the possible effects associated with the propagation of short cracks were considered. The major effects were observed at crack lengths of about 2 mm, but the contributions of short crack phenomena were considered to be small in these experiments, since the two steels were at high strength levels, and short cracks would be expected to be of the order of 10 μm. Also, the two other steels behaved differently and in a way which followed the residual stress patterns. Both M-50NiL and CBS-1000M have a high fracture toughness, with K_lc = 50 MPa · m^1/2 (45 Ksi · in^1/2), and the carburized cases exhibit excellent resistance to rolling contact fatigue. Thus, M-50NiL, carburized, may be useful for bearings where high tensile hoop stresses are developed, since fatigue cracks are slowed in the case by the residual compressive stresses, and fracture is resisted by the relatively tough core.     

Local yielding attending fatigue crack growth

Fatigue crack growth rate measurements were performed at 100°C on an Fe-3Si steel in three thickness conditions and at different ΔK-levels. The test pieces were subsequently sectioned and etched to reveal the plastic deformation attending crack growth both on the surface and in the interior. Unlike preceding studies, the Fe-3Si steel displayed classical cyclic crack growth: well-defined fatigue striations with a spacing close to the per-cycle growth rate, and essentially the same growth rates that have been reported for low and medium strength steels. A highly strained region, approximately one-fifth the size of the monotonie plastic zone, is identified as the cyclic plastic zone. On this basis three regions with distinct cyclic strain histories that precede the crack are identified: a microstrain region wherein the material receives ∼10³ to 10⁴ strain cycles in the range 0 < Δε _P ≲ 10^-3; a cyclic plastic zone corresponding to ∼200 cycles in the range 10^-3 < Δ∈ _P ≲ 10^-1, and a COD-affected zone that receives ∼10 strain cycles in the range 10^-1 ≲ Δ∈ _P ≲ 1. It is suggested that the damage associated with the instabilities in the fatigue substructure to overstrain contribute to the growth mechanism.     

Significance of the small crack growth law and its practical application

The effects of microstructure and specimen size on the fatigue crack growth rate of an annealed 0.42 C steel were investigated under uniaxial fatigue loading in air. Although a dramatic fluctuation of crack growth rate was found in the propagation process of microstructurally small cracks, the mean value of crack growth rate can be evaluated by a simple mechanical parameter, σ _a ⁿ l (l, crack length; n, constant), under high stress levels where small-scale yielding conditions are exceeded. This parameter is also effective for cracks larger than 1 to 2 mm under high stress levels, as long as the finite boundary effect of a specimen on the driving force of crack propagation is considered. The crack growth rate of the alloy was described as a function of stress amplitude and crack length in terms of two mechanical parameters, σ _a ⁿ l and ΔK. The applicable conditions of the two parameters were discussed and manifested.     

Environmental fatigue of an Al-Li-Cu alloy: part I. Intrinsic crack propagation kinetics in hydrogenous environments

Deleterious environmental effects on steady-state, intrinsic fatigue crack propagation (FCP) rates(da/dN) in peak-aged Al-Li-Cu alloy 2090 are established by electrical potential monitoring of short cracks with programmed constant ΔK andK _maxI loading. Such rates are equally unaffected by vacuum, purified helium, and oxygen but are accelerated in order of decreasing effectiveness by aqueous 1 pct NaCl with anodic polarization, pure water’ vapor, moist air, and NaCl with cathodic polarization. Whileda/dN depend on ΔK^4.0 for the inert gases, water vapor and chloride induce multiple power laws and a transition growth rate “plateau.” Environmental effects are strongest at low ΔK. Crack tip damage is ascribed to hydrogen embrittlement because of acceleratedda/dN due to parts-per-million (ppm) levels of H₂O without condensation, impeded molecular flow model predictions of the measured water vapor pressure dependence ofda/dN as affected by mean crack opening, the lack of an effect of film-forming O₂, the likelihood for crack tip hydrogen production in NaCl, and the environmental and ΔK-process zone volume dependencies of the microscopic cracking modes. For NaCl, growth rates decrease with decreasing loading frequency, with the addition of passivating Li₂CO₃ and upon cathodic polarization. These variables increase crack surface film stability to reduce hydrogen entry efficiency. Small crack effects are not observed for 2090; such cracks do not grow at abnormally high rates in single grains or in NaCl and are not arrested at grain boundaries. The hydrogen environmental FCP resistance of 2090 is similar to other 2000 series alloys and is better than 7075. ROBERT S. PIASCIK formerly Graduate Student, Department of Materials Science, University of Virginia.     

Effect of microstructure,strength, and oxygen content on fatigue crack growth rate of Ti-4.5AI-5.0Mo-1.5Cr (CORONA 5)

Fatigue crack growth rate behavior in CORONA 5, an alloy developed for applications requiring high fracture toughness, has been examined for eight material conditions. These conditions were designed to give differences in microstructure, strength level (825 to 1100 MPa [120 to 160 ksi]), and oxygen content (0.100 to 0.174 wt pct), in such a manner that the separate effects of these variables could be defined. For all eight conditions, fatigue crack growth rates (da/dN) are virtually indistinguishable over the full spectrum of stress-intensity range (ΔK) examined,viz., 8 to 40 MPa√m (7 to 36 ksi√in). Concomitantly, it is noted that over the sizable solution annealing range studied (830° to 915 °C [1525° to 1675 °F]), the primary α-phase morphology was substantially invariant. Eachda/dN curve exhibits a bilinear form with a transition point (ΔKT) between 16 and 19 MPa√m (15 and 17 ksi√in). A change in microfractographic appearance occurs at ΔKT, as extensive secondary cracking along α/β interfaces is observed at all hypertransitional levels ofAK, but not for AK < ΔKT. For each material condition, the mean length of primary α platelets is approximately the same as the cyclic plastic zone size at ΔKT. Accordingly, locations ofAKT (and their similarity for the different material conditions) are rationalized in conformance with a cyclic plastic zone model of fatigue crack growth. Finally, the difference in behavior of CORONA 5, as compared to conventional α/β alloys such as Ti-6A1-4V, is rationalized in terms of crack path behavior.     

Fatigue-crack growth in Ti-6Al-4V-0.1Ru in air and seawater: Part I. Design of experiments, assessment, and crack-grown-rate curves

Fatigue-crack growth rates for different simulated ocean environments and loading conditions have been investigated for beta-annealed Ti-6Al-4V-0.1Ru (extra-low interstitials, (ELI)), a candidate material for oil production risers; the focus was on uncovering whether certain combinations of conditions could produce unexpectedly high crack growth rates. A two-level, one-quarter-fraction factorial design-of-experiments (DOE) approach was used to ascertain which testing variables and environmental conditions warranted further study. This study used eight different combinations of variables: parent/deformed material, 27 °C/85 °C temperature, 2 Hz/20 Hz loading frequency, 0.1/0.6 load ratio (R=σ _min/σ _max, where σ _min is the minimum and σ _max is the maximum stress during a fatigue cycle), and aerated/deaerated seawater. Comparisons were based on crack growth rates at ΔK=17 MPa , roughly the middle of the Paris portion of the da/dN vs ΔK curves. The da/dN vs Δ_K curves were also examined, and conclusions based upon these data were compared with those from the DOE. Consideration of the microstructure’s influence on the crack path is postponed until Part II of this article. Samples tested at the higher load ratio showed a statistically significant increase in the crack propagation rate compared to those tested at R=0.1; the same was true of specimens tested at 20 Hz vs those tested at 2 Hz, but the level of significance was lower. The parent material had somewhat higher crack growth rates than the deformed samples. Changes in environmental conditions other than frequency produced little effect on the crack growth rate. Comparison of crack growth rates over the ΔK range measured revealed details that would have not been uncovered in comparisons at a single ΔK value. The Paris exponent ranged between 3.7 and 6.7, and the only systematic variation observed was an increase in the exponent with increasing test frequency. In seawater, cold work (a 5 pct reduction in thickness by rolling) reduced fatigue-crack growth rates by a factor of 2 (compared to the parent material) at intermediate and high ΔK values. There was a crossover of crack growth rates for low ΔK values: below 10 MPa , growth rates were lower for the parent material than for the cold-rolled material, suggesting a higher ΔK _th for the parent material, while above this value, fatigue cracks grew more rapidly in the parent material than in the cold-rolled material. Crack growth rates were slightly higher in seawater than in air, but only slightly more than the sample-to-sample variation of crack growth rates, and cold work reduced fatigue-crack growth rates in air by about the same amount as in seawater. Somewhat more scatter was observed for the R=0.1 tests than for the R=0.6 tests. Differences in temperature (27 °C, 53 °C, and 85 °C) do not appear to affect fatigue-crack growth rates. For ΔK<20 MPa , crack growth rates were similar for 0.2 and 2 Hz but were higher for 20 Hz; above 20 MPa , the crack growth rates were similar for all three frequencies. One explanation for the unusual frequency dependence relies on the possibility that the environment produces different amounts of closure for different test frequencies. According to this view, closure is effective in air and in seawater at 0.2 and 2 Hz but not at 20 Hz: perhaps the higher loading rate breeches the passive layer at a rate more rapid than it can reform. Because the crack growth rate appeared independent of temperature, it is unlikely that there is a significant influence of thermally activated corrosion-fatigue mechanisms for the conditions tested. The results demonstrate that beta-annealed Ti-6V-4Al-0.1Ru (ELI) possesses a robust response to the combinations of environment and loading expected in oil production riser service. The value of the DOE approach was clear, and supplementary tests verified the main effects predicted by the DOE results. Comparison of the single-value DOE results with the da/dN curves reveals a limitation in the former: different slopes of the Paris curves and crossover effects are or would be missed for DOE comparing crack growth rates derived from constant ΔK tests. The use of constant Δσ data and a second level of interrogation, following DOE analysis and based on the da/dN curves, addressed this limitation effectively. A DOE comparison based, for example, on three ΔK values (the lower, middle, and upper portions of the Paris regime) might be another way of proceeding.     

Low cycle fatigue crack propagation in hastelloy-X at 25 and 760 °C

Fatigue crack propagation tests were conducted on Hastelloy-X in air, at 25 °C and at 760 °C under controlled plastic strain amplitudes in the fully plastic low cycle fatigue regime. The crack growth rate data for different strain levels were correlated with the range of theJ integral ΔJ. The ΔJ values were calculated from finite element numerical solutions. It was found that the assumption thatda/dN =A(Δε _ρ ) ^α a is only an approximation of the more general equationda/dN =B(ΔJ) ^α in a narrow range of crack lengths. It is shown that theoretical models predicting low cycle fatigue lives by integrating the fully plastic crack growth rates will be in error if the (da/dN, ΔJ) relationship is not used.     

Variability of large-crack fatigue-crack-growth thresholds in structural alloys

The fatigue threshold of large cracks is known to show substantial variations due to microstructural variability in structural alloys. The ΔK _th variations are dependent on the stress ratio (R); they are extremely large at low R ratios, e.g., R<0.5, but are drastically reduced at high R ratios (R>0.8). The origins of these large variations due to intrinsic and extrinsic mechanisms are examined by theoretical analyses. First, an intrinsic fatigue crack growth (FCG) threshold model is developed for structural alloys by considering the cyclic slip process at the crack tip. Second, the effects of extrinsic mechanisms such as residual plastic stretch, crack deflection, fracture-surface roughness, and oxide wedging are considered both individually and concurrently in order to delineate their relative contributions to threshold variability. The theoretical results indicate that the intrinsic threshold depends on the elastic properties, magnitude of the Burgers vector, yield stress, and Taylor factor (i.e., texture), but is independent of the R ratio or the maximum applied stress intensity factor, K _max. The large variability of ΔK _th at low R ratios and their corresponding dependence on K _max appear to arise from various crack closure mechanisms. Applications of the threshold models to structural alloys show good agreement between theory and experimental data from the literature for steels, Ti, Al, Ni, Cu, Nb, and Mo alloys.     

Fatigue-crack growth in Ti-6Al-4V-0.1Ru in air and seawater: Part II. crack path and microstructure

In Part I of this article, the influence of various testing parameters and environments on the fatigue-crack growth rates in samples of beta-annealed Ti-6Al-4V-0.1Ru (extra-low interstitials) ELI was reported.^[1] A design-of-experiments (DOE) approach was used to survey different combinations of variables, all expected to be important for dynamically loaded offshore oil and gas production risers, and to identify significant effects on the fatigue-crack propagation rate at a stress-intensity range of ΔK=17 MPa . The da/dN vs ΔK curves also were examined for the DOE and supplementary tests, and the results of the two approaches were compared. In this part of the study, the microstructural basis for the robust fatigue-crack growth resistance of beta-annealed Ti-6Al-4V-0.1Ru (ELI) samples was investigated with optical metallography and scanning electron microscopy (SEM). A gradual transition from structure-sensitive (microfacet formation) to structure-insensitive (striation formation) crack propagation centered at ΔK _trans ≅24 MPa , regardless of the combination of testing/environmental conditions examined; the absence of a sharp transition in the slope of the da/dN vs ΔK curves was, therefore, entirely consistent with the fracture-surface morphology. The size of the reversed cyclic plastic zone at the transition ΔK value correlated with the size of the lamella packets, but the number of cycles required to generate a striation ranged between one and ten, suggesting that the crack actively grew over only a portion of its front at any one instant. It is interesting to note that ΔK _trans was the same as the stress range where the da/dN curves at 0.2, 2, and 20 Hz (in seawater) converged to a single curve (refer to Part I of this article): at lower stress ranges, crack growth rates at 20 Hz were significantly higher than those at 0.2 and 2 Hz. The quantitative data showed that fatigue cracks propagated parallel to lamellae interfaces when the long axes of the lamellae made a relatively small angle (<30 deg) to the nominal crack-propagation direction. The crack cut directly across lamellae (i.e., perpendicular to their surfaces) when the long axes of the lamellae were nearly perpendicular to the nominal crack-propagation direction. If the lamellae long axes lay 45 deg to the crack-propagation direction, the crack deflected to run parallel or perpendicular to the lamellae. This behavior occurred regardless of the environment and loading conditions investigated. There was considerable variation in the amount that the cracks deviated from their nominal plane (i.e., the plane normal to the load axis and through the notch tip), with much greater deflections in the cold-rolled than in the parent material, but the angle of the macroscopic crack plane did not exceed 11 deg. Crack branching was observed both at the center and outer surfaces of the samples, regardless of ΔK or other parameters. The relationship between micro- and macrobranches was examined, and branching was more prominent below ΔK _trans, which separated the structure-sensitive and continuum-mode crack-propagation regimes. The relative amounts of micro- and microbranches are reported, and this branching may explain the large scatter in the measurements of the fatigue-crack growth rate often encountered in Ti-6Al-4V and its variants and points to the need for thorough characterization of crack paths, both midplane and surface, as part of the interpretation of da/dN vs ΔK data.     

High frequency stage I corrosion fatigue of austenitic stainless steel (316L)

High frequency (123 Hz) fatigue crack propagation studies were conducted under rising ΔK conditions (R-ratio = 0.22) on single edge notch specimens of austenitic stainless steel (type 316L) that contained an annealed precrack. Tests were conducted in near neutral (pH 5.5) solutions of 1 M NaCl and 1 M NaCl + 0.01 M Na₂S₂O₃ under potentiostatically controlled conditions and in desiccated air. Attention was directed primarily to the near threshold behavior and the stage I (crystallographic) region of cracking. Good mixing between the crack solution and bulk solution was obtained and crack retardation and arrest effects, due to surface roughness induced closure, were minimized at high anodic potentials by electrochemical erosion. Thermodynamic considerations showed that hydrogen played no role in fatigue crack propagation. Analysis of the results in terms of the estimated effective cyclic stress intensity, ΔK _eff, showed a systematic effect of potential on the average crack growth increment per cycle,da/dN. Anodic dissolution processes were considered to make an insignificant contribution toda/dN. A model was proposed for stage I fatigue cracking based on the effect of oxide nucleation rate on restricted slip reversal. The essential features of the model were considered to be relevant to cracking in aqueous environments and in desiccated air.     

Influence of heat treatment on the fatigue crack growth rates of a secondary hardening steel

The relationships between microstructure and fatigue crack propagation behavior were studied in a 5Mo-0.3C steel. Microstructural differences were achieved by varying the tempering treatment. The amounts, distribution, and types of carbides present were influenced by the tempering temperature. Optical metallography and transmission electron microscopy were used to characterize the microstructures. Fatigue fracture surfaces were studied by scanning electron microscopy. For each heat treatment the fatigue crack growth properties were measured under plane strain conditions using a compact tension fracture toughness specimen. The properties were reported using the empirical relation of Paris [da/dN = C_oΔK^m]. It was found that secondary hardening did influence the fatigue crack growth rates. In particular, intergranular modes of fracture during fatigue led to exaggerated fatigue crack growth rates for the tempering treatment producing peak hardness. Limited testing in a dry argon atmosphere showed that the sensitivity of fatigue crack growth rates to environment changed with heat treatment.     

Microstructural effects and crack closure during near

Near threshold fatigue crack growth behavior of a high strength steel under different tempered conditions was investigated. The important aspect of the study is to compare the crack growth behavior in terms of the closure-free component of the threshold stress intensity range, ΔK _th,eff While a systematic variation in the absolute threshold stress intensity range with yield strength was observed, the trend in the intrinsic ΔK _th or ΔK _th,eff exhibited a contrasting behavior. This has been explained as due to the difference in fracture modes during near threshold crack growth at different temper levels. It is shown that in a high strength and high strain hardening microstructure, yielding along crystallographic slip planes is difficult and hence it exhibited a flat transgranular fracture. In a steel with low strain hardening characteristics and relatively low strength, a tendency to crystallographic planar slip is observed consequently resulting in high ΔK _th. Occurrence of a predominantly intergranular fracture is shown to reduce intrinsic ΔK _th drastically and increase crack growth rates. Also shown is that crack closure can occur in high strength steels under certain fracture morphologies. A ‘transgranular planar slip’ during the inception of a ‘microstructure sensitive’ crack growth is essential to promote intergranular and faceted fracture. The occurrence of a maximum in the fraction of intergranular fracture during threshold crack growth corresponds to the ΔK value at which the cyclic plastic zone size becomes equal to the prior austenitic grain size.